From what I've read, they put you in a position where you're wholly reliant upon them. They are your job, your social life, your 'home' life - very difficult to break away once it reaches that point. It's hard enough to quit a regular job because of the upheaval and insecurity - quitting when it will put you on the street and/or have people actively harassing you to come back - yikes.

You're right though, the self identity thing must be huge. You see it with people and their (fairly ordinary) jobs, some people and their 'stuff', they lose the things they think are an intrinsic part of their identity and it's wiggy time.